The present invention relates to a ferrule, made of thermoplastic resin, integrated with a flange into a monolithic structure, for use in an optical fiber connector, and a molding die therefor.
Because conventional ferrules for use in optical fiber connectors are precision parts, such ferrules are manufactured from materials, such as metals, ceramics, and polycrystalline glass, using highly advanced technology including machining, and are thereby expensive products. A ferrule consists of a capillary portion, or capillary, and a flange. As for the capillary, excluding the flange, the injection molding technique is already being utilized by using ceramics, such as zirconia, where it is assumed that the die for the molding is formed to have dimensions approximately 20% greater than the product dimensions of the ferrule, in consideration of shrinkage caused during the ceramic burning process, and that precision working is applied to the capillary after burning. On the other hand, in order for a ferrule to satisfy the performance characteristics required when an optical fiber connector is connected, dimensional accuracy in terms of 1 xcexcm is necessary. When taking into consideration such parameters as the die accuracy, molding characteristics, and burning characteristics, however, it is extremely difficult to satisfy such level of dimensional accuracy, which thereby constitutes a major cause for high manufacturing cost.
Therefore, to reduce the cost, developments and improvements of ferrules are in progress, including those for ferrules applied with injection molding, using thermoplastic resins. A die required for such injection molding may have a structure including a spool 51, a runner 52, a gate 53, and a cavity 54, as shown in FIG. 7, where a ferrule may be molded by casting molten resin into the die to flow through in this order. As shown in FIG. 8, this die is divisible at a parting line 55 into a stationary part fixed to the injection molding machine and a moving part movable in parallel with respect to the stationary part. The die has portions defining a cavity 56 for forming a capillary of the ferrule, and a core pin 57 for forming, in a connected structure, an optical fiber insertion hole including a guide hole part, a conical hole part, and a fine hole part, for inserting an optical fiber. The die also has a portion defining a cavity 58 for engaging with the core pin 57, and a cavity 59 for forming a flange of the ferrule.
Here, it should be noted that, in a ferrule for use in an optical fiber connector, particularly important points may include: the outer diameter of the ferrule; the eccentricity of the axial part of the capillary to the fine hole part of the optical fiber insertion hole; and the state of all the guide hole part, the conical hole part, and the fine hole part, of the optical fiber insertion hole being made free from any protrusions and dents, thereby these holes being arranged in smooth succession. The outer diameter and eccentricity may deeply affect the performance and characteristics of the ferrule when it is assembled into an optical fiber connector, while the smoothness of the optical fiber insertion hole may prevent a fatal defect of insertion failure from occurring.
In the injection molding of a conventional ceramic ferrule, as described above, a stainless steel flange is press fit onto the ceramic (e.g., zirconia) capillary. In this injection molding using ceramics, the die for the molding is formed to have dimensions approximately 20% greater than the product dimensions of the ferrule, and, thereafter, precision post-process working is applied to obtain a ferrule having the product dimensions. This may cause a disadvantage in that material and working expenses would amount high, thereby forcing the cost to rise, and the weight would also get heavier.
Furthermore, in the injection molding of a conventional ceramic ferrule, where die dimensions are greater than the finished product dimensions for machine working the molded parts, the molding die has been employing a construction where a core pin for forming an optical fiber insertion hole including a guide hole part, a conical hole part, and a fine hole part, is integrated into a monolithic structure, and the front end portion of this core pin is inserted into the insertion opening of the cavity for forming the front end surface of the ferrule, and such constructions has caused no problems. In the injection molding using thermoplastic resin, however, because a finished ferrule is formed exactly in the same dimensions as the die dimensions, the core pin 57 tends to break when inserted into the insertion opening of the cavity 58, if the gap between the core pin 57 and the insertion opening is narrow. Therefore, a relatively large gap is required between the core pin 57 and the insertion opening of the cavity 58, which may cause a problem in the eccentricity between the core pin 57 and the insertion opening of the cavity 58 increases, and the core pin 57 may be bent due to pressure fluctuation in the resin when molded. One method to solve the problem could be to divide the core pin 57 between the circular conical part and the straight pin part. This however tends to leave parting marks and level differences in the ferrule after molding, which in turn may cause a new problem in that, when an optical fiber is inserted, the optical fiber is caught by such parting marks and gets broken. Breakage may be particularly likely to occur because thermoplastic resin is softer than optical fiber. Therefore, it is required that the core pin 57 is by all means integrated into a monolithic structure, and concentricity is maintained with high accuracy.
In addition, when forming the axial part of the capillary of the ferrule with the cylindrical cavity 56, using a the thermoplastic resin, a problem may occur in that the front end part of the capillary is expanded to a trumpet-like shape as a result from the total effects of shrinkage and stretch due to quick cooling, pressure rise, and sink mark in the front end surface.
Moreover, when a gate for casting molten resin therethrough is connected to the flange of a complicated shape, fluctuations may occur in the flow and pressure of the molten resin when it passes through the complicated-shaped flange, which in turn may cause pressure fluctuation when the molten resin is cast into the cavity 56 for forming the axial part of the capillary. As a result, a problem occurs in that dispersion is caused to the roundness of the axial part of the capillary.
The present invention is made to solve the problems described above. Therefore, an object of the present invention is to provide a resin ferrule for use in an optical fiber that is capable of mass producing at low cost, reducing weight, preventing the front end part of the capillary from expanding to a trumpet-like shape, securing the roundness of the axial part of the capillary with high accuracy, and, furthermore, preventing eccentricity of the axial part to the optical fiber insertion hole, of the capillary from occurring, and to provide a proper molding die for such resin ferrule.
To solve the problems described above, inventors of the present invention have conducted tests on injection molding dies for various ferrules for use in optical fiber connectors since more than fifteen yeas ago. As a result, it has been known that, when a ring gate is employed, the outer diameter of the capillary of thermoplastic resin ferrules is correlated with the axial length, or width, of this gate, and the pressure preservation capability during the molding, and, thereby, specified dimensions of the targeted areas can be obtained by freely setting these correlated factors. Particularly, it has been found that, by having the flange of a complicated shape equipped with a ring gate of a very large axial width corresponding to xc2xc-xc2xd the outer diameter of the capillary, fluctuations in the flow and pressure of molten resin can be smoothed that are caused by the molten resin flowing from the flange area of a complicated shape into a cavity for forming the capillary. This finding in turn has enabled to conduct highly precise control and stabilization of the outer diameter of the axial part of the capillary.
Also, because the effect of pressure preservation capability from the injection molding machine is outstanding, sink mark due to hourglass-shaped deformation can be prevented that is caused by inner diameter difference between the fine hole part and the guide hole part of the optical fiber insertion hole of the ferrule.
However, because the operation described above uses an axial gate width, close to the product wall thickness of the capillary that is completely different from an ordinary gate width, the operation tends to quickly raise the pressure of the molten resin at the front end surface of the ferrule, and thereby quickly cool the molten resin of a high temperature, which in turn tends to increase the trumpet-shaped deformation occurring in the front end part. Such gate design also may require a tremendous amount of time for the gate working as a post-molding processing. The inventors of the present invention have solved those problems by reducing the quick cooling in the front end surface (slow cooling at the center) and the pressure rise also in the front end surface through equipping a cavity for forming the front end part of the capillary with a concentric circular recess.
It has been also known that the shape of the front end part of the capillary affects the outer diameter of the capillary. However, because the pressure fluctuations within the cavity are smoothed, e.g., because the pressure from the resin to the core pin is applied uniformly in a concentric circular fashion, the trumpet-shaped deformation occurring in the front end part of the capillary has been reduced by equipping the recess in the cavity for forming the front end part of the capillary with a concentric circular shape, without deteriorating the roundness of the axial part of the capillary and the concentricity of the optical fiber insertion hole with the fine hole part. The recess in the cavity may be formed by using a proper shape, such as a spherical or chamfered one, as far as being within the scope of a concentric circular nature, also taking die fabrication easiness into consideration.
In addition, mass production has been made easy by having the ring gate positioned at a place that enables automatic gate processing.
Furthermore, since five years ago, the inventors of the present invention have conducted certain coating tests on thin coating films deposited using the PVD (Physical Vapor Deposition) method, particularly on titanium and aluminum nitride coating films, and have found, as shown in FIG. 5, that film thickness is freely controllable at a film-growing rate of 3 nm/s, and that the coating film is enabled to completely peel off at room temperature, without causing any damage to the cavity. To form such coating film, a process was employed where ceramic thin coating film of titanium and aluminum nitride having hardness of 1800-2300 Hv was formed on the cavity by means of the sputtering method at a low temperature of 573.15 K (300 degrees C.) in plasma, using titanium and aluminum as target materials, in a PVD apparatus where nitrogen gas was first introduced as a reactive gas.
Therefore, according to a first means or aspect of the present invention employed by the inventors of the present invention, based on a result from those tests, a resin ferrule for use in an optical fiber connector is provided including: a capillary having an axial part; and a flange protruding from the axial part; the ferrule having an optical fiber insertion hole axially passing through the capillary, wherein: the capillary and the flange are integrated into a monolithic structure with thermoplastic resin, and the capillary has a planar-shaped front end surface and a front end part extending towards the front end surface, in a convergent fashion where the front end part has the outer diameter of the radial cross-section thereof reduced along the axial direction thereof towards the front end surface with the cross-section kept concentric, thereby resulting in an advantage in that the ferrule is equipped with a highly increased precision, and the ferrule has its material and working costs greatly reduced.
According to a second means or aspect of the present invention, a die is provided for molding a resin, for use in an optical fiber connector, including: a capillary having an axial part; and a flange protruding from the axial part; the ferrule having an optical fiber insertion hole axially passing through the capillary, the capillary and the flange being integrated into a monolithic structure with thermoplastic resin, the capillary having a planar-shaped front end surface and a front end part extending towards the front end surface, the front end part having the outer diameter of the radial cross-section thereof reduced along the axial direction thereof towards the front end surface with the cross-section kept concentric. The die has portions defining: a first cavity for forming the capillary; a second cavity for forming the flange; a core pin for forming the optical fiber insertion hole; an internal hole pin for forming the front end part of the capillary; and a third cavity for engaging with the internal hole pin; the internal hole pin having a recess part for forming the front end part of the capillary, the recess part being formed on an end surface of the internal hole pin on the side facing the first cavity, the third cavity having an internal circumferential surface, the internal circumferential surface having a coating film, made of aluminum and titanium nitride, formed thereon, the internal hole pin being engaged with the third cavity by having the coating film positioned therebetween, thereby resulting in another advantage in that a die is provided that molds the ferrule of the first means with high precision.
According to a third means or aspect of the present invention, a die is provided for molding a resin ferrule, for use in an optical fiber connector, including: a capillary having an axial part; and a flange protruding from the axial part; the ferrule having an optical fiber insertion hole axially passing through the capillary, the capillary and the flange being integrated into a monolithic structure with thermoplastic resin, the capillary having an insertion opening on an end of the optical fiber insertion hole where an optical fiber is inserted thereinto. The die includes portions defining: a first cavity for forming the capillary; a second cavity for forming the flange; and a core pin for forming the optical fiber insertion hole; where the die provided by the third means has only a ring gate, as a gate, that is disposed on an end surface of the second cavity on the side corresponding to the insertion opening, thereby resulting in a further advantage in that ferrules having an outer diameter of high precision are enabled to be mass produced stably.
According to a fourth means or aspect of the present invention, a die is provided as specified either by the second or third means of the present invention described above, wherein the core pin is integrated into a monolithic structure, whereby the optical fiber insertion hole is made free from any parting marks, resulting in an additional advantage in that the optical fiber is kept free from any damage during the insertion.
Other objects and advantages of the present invention may become more apparent by referring to the following detailed description and accompanying drawings briefly described below.